The present invention relates generally to techniques for displaying computer information on televisions.
In a number of applications, users may wish to display computer information on a conventional television receiver. Using a television receiver (which the user may already have) instead of a computer monitor may result in cost savings. In addition, some users may wish to have a computer system which doubles as a television receiver. Thus, the user may watch conventional television programs using a tuner card in the computer system to tune to the desired programs. In devices known as set top computers, a computer system may be mounted atop a conventional television to implement combined television receiving and conventional computer functions. For example, such systems advantageously implement electronic programming guides for computer control of television program access.
Interlacing contributes to the inferior quality of images developed by computer systems that are displayed on television monitors. Interlacing is a technique by which a complete television picture is developed in two passes from top to bottom on the television screen. With interlacing, the first pass paints all the “odd” lines and the second pass paints the “even” lines.
Flicker may occur when the images in the odd lines are very different from the images in the even lines. As the odd and even lines are alternately displayed, the eye may perceive the quick appearing and disappearing of visual information. Flicker is especially noticeable when viewing thin horizontal lines that only take up a single odd or even row. The same objectionable phenomenon occurs at the horizontal boundaries of text.
A flicker filter may be used to reduce such flickering. A flicker filter is generally a 3 or 5 tap finite impulse response (FIR) filter. An N-tap filter uses N multiplications and N-1 additions per color component of each output pixel. An equation representing the calculation implemented by a 3-tap filter is as follows:Pout=T1*P1+T2*P2+T3*P3T1, T2 and T3 are the tap values and P1, P2 and P3 are the values of adjacent pixels. Thus, flicker filters are quite computationally intensive.
Some flicker filters work by altering the information in the odd and even lines of a television picture so that the alternating lines are more similar to each other. In this way, when the lines appear and disappear in the interlacing process, the flicker is less noticeable. The more similar the lines are made to appear, the less flicker is visible. Other flicker filters reduce flicker by completely discarding every other line of the image, and displaying two successive passes of the remaining lines. Because the interlaced lines are now identical, flicker is less visible. In either case, vertical resolution is sacrificed. The obvious trade-off is that as flicker is reduced, more and more information is altered or lost from the original picture. Thin, horizontal lines may disappear. Small text may become unreadable.
The quality of flicker filters may vary based on design and cost. Low end flicker filters apply their techniques to the entire screen indiscriminately or blindly. These types of flicker filters may cause undesirable results, when only a small portion of the screen needs filtering, by adversely altering regions that were unnecessarily filtered.
Thus, there is a continuing need for ways to economically improve the display of information from processor-based systems on televisions.